1. Field of the Invention
The present invention generally relates to electric current responsive actuators such as circuit breakers, contactors and other electrical distribution and control devices. More particularly it relates to an improved actuator in which electromagnetic and thermal actuating elements are coupled together to produce a resultant actuation force which enhances the thermal tripping characteristics of the actuator.
2. Description of the Prior Art
Circuit breakers and other electric current responsive distribution and control devices in the prior art employ separate thermal and magnetic tripping mechanisms. Typically, the thermal tripping mechanism responds to currents slightly in excess of nominal which persist for a relatively long interval and the magnetic tripping mechanism responds in a short interval to currents substantially in excess of nominal. At current levels below the level for magnetic actuation, the trip characteristic is a function of the product of the square of the current and time. At modest overcurrents, the overcurrent which passes through before tripping is determined by the type and size of the thermal elements used. As a practical matter this limits the thermal response which can be realized with prior art actuators.
U.S. Pat. No. 4,275,370 to Sims discloses an electrical overload circuit breaker which uses a solenoid as the current-responsive tripping mechanism. The solenoid coil is a helical spring made of a shape memory alloy. The shape-memory alloy spring mechanically acts upon the solenoid armature. Current passing through the dual function shape-memory alloy spring and solenoid coil urges the armature both mechanically and electromagnetically in tripping direction. While providing a compact unit, the Sims breaker is designed so that mechanical and electromagnetic forces act essentially independently to trip the breaker in response to an over-current condition. In the Sims device, with currents in a range of about 1.5 to 4 times nominal current, the electromagnet force is not sufficient to trip an over-center toggle and tripping of the toggle is effected by a prolonged over current of sufficient duration to result in a temperature rise in the shape memory effect spring through its transition temperature range. When a massive overload (e.g., 4 to 9 times nominal current) occurs, the high current through the helical spring generates a large magnetic force on the armature which trips the toggle. In the massive overload situation, any force generated by the shaped memory effect of the spring takes little part in the tripping action.